The present invention relates to an alumina-carbon based refractory, especially a slide gate refractory, having high resistance to corrosion and spalling.
Heretofore, there have been used to produce slide gate plates such refractory materials as an alumina based material, magnesia-spinel based material and alumina-carbon based material.
However, recently the application of these conventional refractory materials has become restricted because of the increased proportion of casting corrosive steel-type materials such as Ca-treated steel. In particular there is a growing need of high-resistant slide gate refractory materials.
Following various attempts have been made to improve the resistance of refractories to thermal shock.
One attempt is to include SiO.sub.2 in a refractory material. This is effective to prevent the occurrence of small cracks.
It has been proposed that the thermal expansion of an alumina-carbon based refractory be reduced by addition of a low thermal expansion aggregate such as mullite or mullite-zirconia. This method is described in, for example, Japanese Patent Application Unexamined Publication No. 165549/1981. However, this method cannot provide sufficient resistance to corrosion because of the inclusion of SiO.sub.2.
It has also been proposed that monoclinic zirconia be added to a refractory to develop microcracks by the crystal transformation. This method is described in, for example, Japanese Patent Application Unexamined Publication No. 20777/1983. However, this method has such a problem that microcracks occur in binding textures themselves in a refractory, thereby reducing binding strength.
Japanese Patent Application Examined Publication No. 2620/1986 proposes a process for producing an alumina-carbon based sliding nozzle plate which comprises providing 5-90 wt % of a refractory material whose chemistry consists of 80-98 wt % of Al.sub.2 O.sub.3 and 2-20 wt % of ZrO.sub.2, mixing said refractory material with other raw materials, shaping the mixture, and firing the shaped mixture. The alumina-carbon based refractories produced by this conventional method have suffered from the disadvantage that their flexural strength, corrosion resistance and anti-spalling property are sometimes insufficient depending on the conditions of their use.
If the alumina-carbon based refractory includes SiO.sub.2, compounds of low melting temperature (i.e., SiO.sub.2 --CaO--FeO, SiO.sub.2 --FeO--MnO and SiO.sub.2 --FeO--Al.sub.2 O.sub.3) are likely to be generated through the chemical reaction between SiO.sub.2 and specific components in steel. In particular, SiO.sub.2 in a slide gate refractory remarkably reacts with Ca in Ca-treated steel. This becomes a cause of reducing resistance of the refractory to corrosion. Since the corrosion quantity of the slide gate refractory is approximately proportional to the SiO.sub.2 content in the refractory, increase of the SiO.sub.2 content means increase of the corrosion. On the other hand, the reduction of SiO.sub.2 content produces another problem of deterioration in the anti-spalling property. Appearance of the refractory has been desired which is sufficient both in the corrosion resistance and in the anti-spalling property.